Apparatus and method for regenerating electrolyte of a redox flow battery

ABSTRACT

Deteriorated positive electrolyte containing Fe 3  /Fe 2+  ions for a redox flow battery is regenerated in a negative electrolyte chamber of an electrolyte regenerating apparatus. For this purpose, a formic acid solution is introduced into a positive electrolyte chamber of the electrolyte regenerating apparatus and a voltage is applied between a negative electrode of the negative electrolyte chamber and a positive electrode of the positive electrolyte chamber, whereby excessive Fe 3+  ions in the deteriorated positive electrolyte are changed to Fe 2+  ions for regeneration. The formic acid is changed into carbon dioxide which is not noxious. All gases generated by the regeneration are not noxious, whereby the regeneration can be carried out with safety.

FIELD OF THE INVENTION

The present invention relates to an apparatus and a method forregenerating electrolyte of a redox flow battery and particularly to anelectrolyte regenerating apparatus for a redox flow battery, which issafe and easy to handle.

BACKGROUND INFORMATION

Electric power companies must take into account the demand for electricpower in order to generate and supply stable electric power toconsumers. For this purpose, electric power companies always makeefforts to have power stations for satisfying the largest demand, sothat electricity can be generated according to the demand. However, asshown by the power demand curve A in FIG. 1, there is a large differencebetween a demand for power at daytime and that at night time. Similarphenomena can be observed for different time periods such as weeks,months or seasons.

Therefore, if it is possible to store electric power with highefficiency, surplus power, corresponding to the portion shown by X inFIG. 1, can be stored at an off-peak time and supplied at a peak time sothat the portion shown by Y in FIG. 1 may be made up for by such storedsurplus power. In such a manner, the supply of power can be made tocorrespond to changes in demand and the electric companies need onlygenerate an almost constant power quantity corresponding to the brokenline Z in FIG. 1 at all times. If such capacity load leveling can beaccomplished, it is possible to allow the electric utilities to maximizethe use of the most efficient base load plants.

Therefore, various power storage methods have been proposed. Forexample, a pumped hydro storage power generation method has been alreadypractically utilized. According to the pumped hydro storage powergeneration method, a power station is located at a considerable distancefrom the place where power is consumed. Consequently, this method hasdisadvantages such as transmission and transformation losses and limitedlocations for the stations in view of environmental conditions. Inconsequence, it is desired to develop new power storing techniques inplace of the pumped hydro storage power generation method. Redox flowbatteries are being developed as one of such techniques.

FIG. 2 is a schematic structural view of an example of an alreadyproposed redox flow battery. This redox flow battery 1 comprises a cell2, a positive electrolyte tank 3 and a negative electrolyte tank 4.Since the two tanks 3 and 4 are used, this battery is called a 2-tanktype battery. The cell 2 is separated by a membrane formed by an ionexchange membrane 5 for example, so that a positive electrode cell 2aand a negative electrode cell 2b are formed by the separation. Apositive electrode 6 is provided in the positive electrode cell 2a,while a negative electrode 7 is provided in the negative electrode cell2b.

The positive electrode cell 2a and the positive electrolyte tank 3 areconnected through first and second pipes 11 and 12. The negativeelectrode cell 2b and the negative electrolyte tank 4 are connectedthrough third and fourth pipes 13 and 14. A positive electrolyte isintroduced as a reaction solution into the positive electrolyte tank 3and a negative electrolyte is introduced as a reaction solution into thenegative electrolyte tank 4. A pump P2 as reaction solution supply meansis provided in the first pipe 11, while a pump P1 is provided in thesecond pipe 13. The positive and negative electrolyte react in thepositive and negative electrode cells 2a and 2b, respectively. Thepositive electrolyte after the reaction returns into the positiveelectrolyte tank 3 through the second pipe 12, while the negativeelectrolyte after the reaction returns into the negative electrolytetank 4 through the fourth pipe 14.

In the redox flow battery shown in FIG. 2, a solution of ions such asiron ions having a variable valence is used as the positive electrolyteand a solution of ions such as chromium ions having a variable valenceis used as the negative electrolyte.

If hydrochloric acid solution containing a positive reactant Fe³⁺ /Fe²⁺is used as the positive electrolyte and hydrochloric acid solutioncontaining a negative reactant Cr³⁺ /Cr³⁺ is used as the negativeelectrolyte, reactions at the positive electrode 6 and at the negativeelectrode 7 are as follows: ##STR1##

An electromotive force of about 1 volt is obtained by theelectrochemical reactions represented by the above formulas.

However, in reality, the above mentioned electrochemical reactions donot proceed equally at the positive and negative electrodes 6 and 7 asdescribed above. This phenomenon is considered to be caused by the sidereactions described below.

First, hydrogen gas is generated at the negative electrode at the end ofa charging period and, as a result, an absolute quantity ofoxidation-reduction pairs (Cr³⁺ /Fe²⁺ or Cr²⁺ Fe³⁺) is decreased.

Secondly, Cr²⁺ ions are relatively unstable and are liable to beoxidized by oxygen in the air and thus they are easily changed to Cr³⁺ions. In such a case, the absolute quantity of oxidation-reduction pairscaused by the battery reactions is also decreased.

If the above described side reactions occur and the absolute quantity ofoxidation-reduction pairs is decreased, electric energy stored in thebattery, that is, the battery capacity is decreased as a result of arepetition of charging and discharging operations. Further, the internalresistance of the battery is increased and the charge and dischargeefficiency often deteriorates.

In order to solve the above described problems, an apparatus forregenerating electrolyte of a redox flow battery has been disclosed inJapanese Patent Laying-Open No. 304580/1988.

FIG. 3 is a schematic diagram showing a construction of the apparatusfor regenerating electrolyte of a redox flow battery described inJapanese Patent Laying-Open No. 304580/1988.

Referring to FIG. 3, a positive electrolyte tank 3 of the redox flowbattery 1 is connected with the electrolyte regenerating apparatus 16 ofthe redox flow battery 1. Since the components of the redox flow battery1 are the same as shown in FIG. 2, the description is not repeated.

The electrolyte regenerating apparatus 16 has a positive electrolytechamber 20 and a negative electrolyte chamber 22 separated by a membrane18. The positive electrolyte chamber 20 has a positive electrode 24 andthe negative electrolyte chamber 22 has a negative electrode 26. Voltageapplying means (not shown) for applying a voltage to the positive andnegative electrodes 24 and 26 is connected to the positive and negativeelectrodes 24 and 26.

A gas-liquid separator 28 and a hydrochloric acid solution tank 30 areconnected to the positive electrolyte chamber 20. Hydrochloric acidsolution is supplied from the hydrochloric acid solution tank 30 intothe positive electrolyte chamber 20 by means of a pump 32. Thegas-liquid separator 28 separates chlorine gas generated in the positiveelectrolyte chamber 20, from the hydrochloric acid solution. A chlorinegas absorbing device 34 for absorbing the separated chlorine gas isconnected to the gas-liquid separator 28.

The negative electrolyte chamber 22 is connected to the positiveelectrolyte tank 3 of the redox flow battery 1 so that the positiveelectrolyte in the positive electrolyte tank 3 of the redox flow battery1 is supplied into the negative electrolyte chamber 22 and is dischargedfrom the negative electrolyte chamber 22 to the positive electrolytetank 3.

The above described electrolyte regenerating apparatus operates asfollows when using Fe ³⁺ /Fe²⁺ ions as the positive reactant and Cr³⁺/Cr²⁺ ions as the negative reactant.

While charging and discharging operations are repeated in the redox flowbattery 1, the amount of Fe³⁺ ions (or Cr³⁺ ions) of theoxidation-reduction pairs becomes excessive causing a deterioration ofthe electrolyte as described above. In this electrolyte regeneratingapparatus, the excessive Fe³⁺ ions are reduced by using the electrolyteregenerating apparatus 16 as described below. Accordingly, the Fe²⁺ ionsare regenerated and a normal balance of the oxidation-reduction pairs ismaintained.

More specifically, the positive electrolyte supplied from the positiveelectrolyte tank 3 of the redox flow battery 1 to the negativeelectrolyte chamber 22 of the electrolyte regenerating apparatus 16reacts according to the below indicated formula (1) when a voltage isapplied to the electrodes 24 and 26.

    Fe.sup.3+ +e.sup.- →Fe.sup.2+                       (1)

Hydrochloric acid supplied from the hydrochloric acid solution tank 30to the positive electrolyte chamber 20, reacts according to the belowindicated formulas (2) and (3).

    Cl.sup.- →1/2Cl.sub.2 +e.sup.-                      (2)

    1/2H.sub.2 O 1/40.sub.2 +H.sup.+ +e.sup.-                  (3)

Consequently, in the electrolyte regenerating apparatus 16, Fe³⁺ ionsare reduced to Fe²⁺ ions at the negative electrode 26 and chlorine gasas well as oxygen gas are generated at the positive electrode 24. Thischlorine gas is separated from the hydrochloric acid solution by thegas-liquid separator 28 and it is absorbed by the chlorine gas absorbingdevice 34. When the positive electrolyte containing the Fe²⁺ ionsreduced by the electrolyte regenerating apparatus 16 is returned to thepositive electrolyte tank 3 of the redox flow battery 1, the quantity ofthe oxidation-reduction pairs in the redox flow battery 1 is restored tothe initial value.

If the electrolyte regenerating described above is connected to a redoxflow battery, the balance of the oxidation-reduction pairs of theelectrolyte of the redox flow battery is maintained normal and thebattery capacity is restored.

However, since hydrochloric acid solution is used as a solution forelectrochemically regenerating the positive electrolyte of the redoxflow battery 1, noxious chlorine gas is generated in the positiveelectrolyte chamber, causing a safety hazard. In a practical use, safetycan be ensured if the gas absorbing device 34 is provided. However, insuch a case, the apparatus has a large size and the handling thereofbecomes complicated.

In addition, since chlorine gas is treated, the apparatus needs to bemade of a corrosion-resistant material. Furthermore, in order to preventleakage of chlorine gas, it is necessary to take effective measures forair tightness of the apparatus. As a result, the apparatus is veryexpensive.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide anapparatus for regenerating electrolyte of a redox flow battery, in whichthe battery capacity of the redox flow battery can be restored to aninitial capacity value.

Another object of the present invention is to provide an apparatus forregenerating electrolyte of a redox flow battery, in which the batterycapacity of the redox flow battery can be restored to an initialcapacity value without generation of noxious gas.

Still another object of the present invention is to provide an apparatusfor regenerating electrolyte of a redox flow battery, which is safe tooperate and easy to handle.

A further object of the present invention is to provide an apparatus forregenerating electrolyte of a redox flow battery, which can be madecompact.

A still further object of the present invention is to provide anapparatus for regenerating electrolyte of a redox flow battery, whichcan be manufactured with inexpensive materials.

A still further object of the present invention is to provide anapparatus for regenerating electrolyte of a redox flow battery, which issafe in operation and in which electrodes used in the apparatus are notdeteriorated.

A still further object of the present invention is to provide a methodfor regenerating electrolyte of a redox flow battery, making it possibleto recover a battery capacity of the redox flow battery to an initialcapacity value without generating noxious gas.

A still further object of the present invention is to provide a methodfor regenerating electrolyte of a redox flow battery, in which a batterycapacity can be maintained constantly at an initial capacity value.

In order to accomplish the above described objects, an apparatus forregenerating electrolyte of a redox flow battery according to thepresent invention includes a negative electrolyte chamber and a positiveelectrolyte chamber. Positive electrolyte of the redox flow battery isintroduced into the negative electrolyte chamber from a positiveelectrolyte tank of the battery through an inlet pipe. The negativeelectrolyte chamber has a negative electrode immersed in the positiveelectrolyte. The positive electrolyte chamber is connected to thenegative electrolyte chamber through a membrane and it contains a formicacid solution and a positive electrode immersed in this formic acidsolution. The electrolyte regenerating apparatus according to thepresent invention further includes voltage applying a means for applyinga voltage between the positive for negative electrodes and emittingelectrons from the above mentioned formic acid solution so that aportion of the metal ions in a high valence state contained in thepositive electrolyte, can be changed to metal ions in a low valencestate. This apparatus further includes an outlet pipe for returning thepositive electrolyte containing the metal ions in the low valence state,to the above mentioned positive electrolyte tank.

In the present electrolyte regenerating apparatus the concentration ofthe formic acid solution cannot be generally limited because it dependson conditions of use of the battery, but the formic acid solution isnormally a solution containing formic acid of more than 40 wt.% and morepreferably a solution containing formic acid of more than 80 wt.%. Ifthe concentration is lower than the above mentioned lowest limit, theportion of the positive electrode used in the electrolyte regeneratingapparatus deteriorates, causing unfavorable effects.

According to a method for regenerating electrolyte of a redox flowbattery according to another aspect of the invention, a positiveelectrolyte of the redox flow battery is introduced into a negativeelectrolyte chamber having a negative electrode, through an inlet pipe.Then, a formic acid solution is introduced into a positive electrolytechamber having a positive electrode and connected to the negativeelectrolyte chamber through a membrane. Thereafter, voltage is appliedbetween the positive and negative electrodes whereby electrons areemitted from the formic acid solution so that a portion of the metalions in a high valence state contained in the positive electrolyte, arechanged to metal ions in a low valence state. Then, the above mentionedpositive electrolyte containing the metal ions in the low valence state,is returned to the redox flow battery.

In the negative electrolyte chamber of the electrolyte regeneratingapparatus according to the present invention, an electrochemicalreaction as indicated below occurs.

    Fe.sup.3+ +e.sup.- →Fe.sup.2 +                      (4)

Thus, Fe³⁺ ions are changed to Fe²⁺ ions, whereby the quantity ofoxidation-reduction pairs reattains an initial value.

Furthermore, in the positive electrolyte chamber of the electrolyteregenerating apparatus according to the present invention,electrochemical reactions as indicated below occur.

    1/2 HCOOH→1/2CO.sub.2 +H.sup.+ +e.sup.-             (5)

    1/2 H.sub.2 O→H.sup.+ +1/40.sub.2 +e.sup.-          (6)

Referring to the above indicated formulas (5) and (6), the gasesgenerated by the electrochemical reactions are carbon dioxide gas andoxygen gas. Since those carbon dioxide gas and oxygen gas are notnoxious, they are easy to handle and safety is ensured even if a humaninhales these gases.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a power demand curve.

FIG. 2 is a schematic structural view of an example of a conventionalredox flow battery.

FIG. 3 is a schematic view of a redox flow battery including aconventional electrolyte regenerating apparatus.

FIG. 4 is a schematic view of a redox flow battery including anelectrolyte regenerating apparatus according to the present invention.

FIG. 5 is a graph showing the relationship between the number ofcharge/discharge cycles and a battery capacity in a redox flow batteryincluding an electrolyte regenerating apparatus according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS AND OF THE BEST MODEOF THE INVENTION

An embodiment of the present invention will be described in thefollowing. However, the present invention is not limited thereto.

FIG. 4 is a schematic block diagram for explaining an embodiment of theinvention, wherein a redox flow battery 1 comprises a cell 2, a positiveelectrolyte tank 3 and a negative electrolyte tank 4. The cell 2 isseparated by a membrane 5 of an ion exchange membrane for example, toform a positive electrode cell 2a in one portion and a negativeelectrode cell 2b in the other portion. A positive electrode 6 isprovided in the positive electrode cell 2a and a negative electrode 7 isprovided in the negative electrode cell 2b. The positive electrode cell2a and the positive electrolyte tank 3 are connected by a first pipe 11and a second pipe 12. On the other hand, the negative electrode cell 2band the negative electrolyte tank 4 are connected by a third pipe 13 anda fourth pipe 14. Positive electrolyte containing a positive reactantsuch as iron ions, the valence of which changes, is stored in thepositive electrolyte tank 3. Negative electrolyte containing a negativereactant such as chromium ions, the valence of which changes is storedin the negative electrolyte tank 4. The positive electrolyte is suppliedto the cell 2a by a pump 15 provided in the first pipe 11. The negativeelectrolyte in the negative electrolyte tank 4 is supplied to thenegative electrode cell 2b by a pump 17 provided in the third pipe 13.The positive electrolyte supplied to the positive electrode cell 2areacts in the cell 2a and the solution after having reacted, returnsinto the positive electrolyte tank 3 through the second pipe 12. Thenegative electrolyte supplied to the negative electrode cell 2b reactsin the cell 2b and the solution after having reacted, returns into thenegative electrolyte tank 4 through the fourth pipe 14.

An electrolyte regenerating apparatus 16 comprises a positiveelectrolyte chamber 20 and a negative electrolyte chamber 22. Thepositive electrolyte chamber 20 and the negative electrolyte chamber 22are connected through a membrane 18. A positive electrode 24 is providedin the positive electrolyte chamber 20, while a negative electrode 26 isprovided in the negative electrolyte chamber 22. The positiveelectrolyte tank 3 and the negative electrolyte chamber 22 are connectedby an inlet pipe line 38 and an outlet pipe line 39. A pump 40 isprovided in the inlet pipe line 38. The pump 40 is used to circulate thepositive electrolyte stored in the positive electrolyte tank 3 through acirculation path including the positive electrolyte tank 3, the inletpipe line 38, the negative electrolyte chamber 22 and the outlet pipeline 39. Voltage applying means, merely shown symbolically, apply avoltage between the positive and negative electrodes 24 and 26 so that aportion of the Fe³⁺ ions contained in the positive electrolyte can bechanged to Fe²⁺ ions.

The positive electrolyte chamber 20 is connected to a formic acidsolution tank 41 through a supply pipe line 36 and a recycle pipe line37. The supply pipe line 36 sends formic acid solution from the formicacid solution tank 41 into the positive electrolyte chamber 20. Therecycling pipe line 37 introduces the formic acid solution dischargedfrom the positive electrolyte chamber 20, into the formic acid solutiontank 41. Gas-liquid separating means 28 for separating carbon dioxidegas from the received formic acid solution is provided in the recyclingpipe line 37. This gas-liquid separating means 28 is provided ifrequired but it is not an essential element for the present invention. Apump 32 is provided in the supply pipe line 36. The pump 32 is used tocirculate the formic acid solution stored in the formic acid solutiontank 41 through a circulation path including the formic acid solutiontank 41, the supply pipe line 36, the positive electrolyte chamber 20,the recycle pipe line 37 and the gas-liquid separating means 28 providedin the recycling pipe line 37.

In the following, the present method for regenerating electrolyte of theredox flow battery using the above described electrolyte regeneratingapparatus will be described. In the following description, Fe³⁺ /Fe²⁺ions are used as the positive reactant and Cr³⁺ /Cr²⁺ ions are used asthe negative reactant.

First, while charging and discharging operations are repeated in theredox flow battery 1, the amount of Fe³⁺ ions, or Cr³⁺ ions of theoxidation-reduction pairs becomes excessive and the electrolytedeteriorates as described above.

Because of the deterioration of the electrolyte, the positiveelectrolyte having excessive Fe³⁺ ions, is supplied from the positiveelectrolyte tank 3 to the negative electrolyte chamber 22. Then, theformic acid solution is supplied from the formic acid solution tank 41to the positive electrolyte chamber 20. Thereafter, voltage is appliedbetween the positive and negative electrodes 24 and 26. Then, anelectrochemical reaction indicated by the following formula (7) occursin the negative electrolyte chamber 22 and Fe³⁺ ions are changed to Fe²⁺ions.

    Fe.sup.3+ +e.sup.- →Fe.sup.2+                       (7)

Hereafter, the positive electrolyte passes through the outlet pipe line39 and returns into the positive electrolyte tank 3. Thus, a normalbalance is regained in the oxidation-reduction pairs of the electrolyteof the redox flow battery 1 and the battery capacity of the redox flowbattery 1 is restored to the initial capacity value.

On the other hand, an electrochemical reaction indicated by thefollowing formula (8) occurs in the positive electrolyte chamber 20 andcarbon dioxide and oxygen are generated.

    1/2 HCOOH→1/2CO.sub.2 +H.sup.+ +e.sup.-             (8)

    1/2 H.sub.2 O→1/40.sub.2 +H.sup.+ +e.sup.-          (9)

Since the formic acid solution circulates in the circulation pathincluding the formic acid solution tank 41, the supply pipe line 36, thepositive electrolyte chamber 20, the recycle pipe line 37, and thegas-liquid separator 28 provided in the recycle pipe line 37, the carbondioxide gas and oxygen gas generated by the above indicatedelectrochemical reaction, are separated by the gas-liquid separator 28.Since the carbon dioxide gas and oxygen gas are not noxious, they can beemitted directly into the air and a special gas absorbing device 28 isnot required. Consequently, the apparatus can be made compact. Further,since the carbon dioxide gas and oxygen gas are safe for the human body,the apparatus does not need to be airtight and thus, the apparatus iseasy to handle and to operate. In addition, since the apparatus does notneed to be made of a special material, it has an economical advantage.In the apparatus according to the present invention, it is onlynecessary to supply formic acid and water periodically by amountscompensating for the generations of carbon dioxide gas and oxygen gasand thus the operation of the apparatus is simple.

EMBODIMENT 1

An experiment was carried out by using a redox flow battery having anelectrode area of 1500 cm² and an electrolyte regenerating apparatushaving an electrode area of 1500 cm². A cation exchange membrane wasused as a membrane material in the cells of the redox flow battery. Acation exchange membrane was used also for the cells of the electrolyteregenerating apparatus. Combinations of carbon fiber cloth and graphiteplates were used for the positive and negative electrodes. Theconditions for the experiment were as follows.

(1) In the redox flow battery

1. electrolyte

positive electrolyte: a solution obtained by dissolving 1 mol of FeCl₂into 3NHCl

negative electrolyte: a solution obtained by dissolving

1 mol of CrCl₃ into 3NHCl,

2. current density: 40 mA/cm²

constant current charging and discharging of the formed-under the abovementioned were repeated 50 times under the above mentioned conditionswhereby the initial discharge enabling time of 100 minutes was changedto 75 minutes. The electrolyte thus deteriorated were used for aregeneration test.

(2) In the electrolyte regenerating apparatus

1. electrolyte regenerating liquid: formic acid solution, having aconcentration of 80 %

2. current density: 40 mA/cm²

Under the above indicated conditions, the electrolyte regeneratingapparatus was operated, whereby carbon dioxide gas and oxygen gas weregenerated and the redox flow battery was regenerated as a battery havinga discharge enabling time of about 100 minutes.

EMBODIMENT 2

Under the same conditions as those in the above embodiment 1, adeteriorated electrolyte was prepared. Under the same conditions asthose in the embodiment 1, except the use of a 40 % formic acid solutionas the electrolyte regenerating liquid, the electrolyte regeneratingapparatus was operated. As a result, in the same manner as in theembodiment 1, carbon dioxide gas and oxygen gas were generated and theredox flow battery was regenerated as a battery having a dischargeenabling time of about 100 minutes.

However, after the electrolyte regenerating apparatus was repeatedlyoperated in this case, deterioration of the positive electrode 24 wasobserved by simple visual inspection. On the other hand, in the case ofthe embodiment 1, if the electrolyte regenerating operation wasrepeatedly carried out, deterioration of the positive electrode 24 wasnot observed. Those results of the experiments indicate that with alarge quantity of water, the reaction of the formula (9) occurs morefrequently to promote deterioration of the electrode due to oxygengenerated by the reaction. Therefore, the concentration of formic acidis preferably at a high value.

EMBODIMENT 3

Under the conditions of the embodiment 1, two experiments were carriedout. One of the experiments was carried out by a method of operating theredox flow battery while stopping the operation of the electrolyteregenerating apparatus. The other experiment was carried out by a methodof operating the electrolyte regenerating apparatus while operating theredox flow battery. FIG. 5 shows the results of these experiments,namely, a graph showing the battery capacity in % as a function of thenumber of charging and discharging cycles. The line A in FIG. 5represents the result in the case of operating the electrolyteregenerating apparatus while operating the redox flow battery. The lineB represents the result in the case of operating the redox flow batterywhile stopping the operation of the electrolyte regenerating apparatus.Comparing the lines A and B, it is understood that the battery capacityof the redox flow battery is constantly maintained at the initialcapacity value if the electrolyte regenerating apparatus is operatedwhile the redox flow battery is operated.

As described above, according to the present invention, a formic acidsolution is used as the electrolyte regenerating liquid and accordinglygases generated by the regeneration of the electrolyte are carbondioxide gas and oxygen gas. Since the carbon dioxide gas and oxygen gasare not noxious, they are safe for the human body and they can bedischarged directly into the air. Consequently, a gas absorbing deviceas used in a conventional electrolyte regenerating apparatus is notrequired. As a result, the apparatus can be made compact. Further, sincethe generated carbon dioxide gas and oxygen gas are not noxious for thehuman body, the apparatus does not need to be made airtight andtherefore the manufacturing of the apparatus is easy. In addition, sincethe electrolyte regenerating apparatus does not need to be made of anexpensive corrosion-resistant material, the manufacturing cost can bereduced. Moreover, the maintenance work becomes very simple because itis only necessary in the apparatus according to the present invention tosupply periodically formic acid and water by amounts compensating forthe generation of carbon dioxide gas and oxygen gas. Contrary thereto itwas necessary in a conventional apparatus to the sodium hydroxide in thegas absorbing container and to complementarily supply hydrochloric acidas chlorine gas was generated.

According to the method of regenerating electrolyte of the redox flowbattery according to the present invention, generated gases are notnoxious carbon dioxide gas and oxygen gas and those gases cause no harmto the human body. Accordingly, electrolyte regenerating operation canbe carried out with safety.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A battery system comprising a redox flow batteryand a regenerating cell for regenerating an electrolyte of said redoxflow battery, said regenerating cell (16), comprising: a negativeelectrolyte chamber (22) containing a positive electrolyte of said redoxflow battery (1) introduced from a positive electrolyte tank (3) of saidredox flow battery (1) through an inlet pipe, said negative electrolytechamber (22) having a negative electrode (26) immersed in said positiveelectrolyte, a positive electrolyte chamber (20) connected to saidnegative electrolyte chamber through a membrane (18), said positiveelectrolyte chamber containing a formic acid solution and having apositive electrode (24) immersed in said formic acid solution, voltageapplying means for applying a voltage between said positive and negativeelectrodes (24, 26) to emit electrons from said formic acid solution forchanging a portion of metal ions contained in said positive electrolyteand having a high valence state, to metal ions having a lower valencestate, and an outlet pipe for returning, to said positive electrolytetank, said positive electrolyte containing low valence state metal ionsproduced by an application of a voltage by said voltage applying meansto said positive and negative electrodes (24, 26).
 2. The battery systemof claim 1, further comprising: a formic acid solution tank for storingsaid formic acid solution to be supplied to said positive electrolytechamber, a supply pipe for sending the formic acid solution from saidformic acid solution tank to said positive electrolyte chamber, and areturn pipe for recycling said formic acid solution discharged from saidpositive electrolyte chamber, to said formic acid solution tank.
 3. Thebattery system of claim 2, further comprising: gas-liquid separatingmeans connected in said return pipe, for separating carbon dioxide gasfrom said formic acid solution.
 4. The battery system of claim 1,wherein said formic acid solution is a water solution containing formicacid of 40 wt.% or more.
 5. The battery system of claim 4, wherein saidformic acid solution is a water solution containing formic acid of 80wt.% or more.
 6. A method for regenerating an electrolyte of a redoxflow battery, comprising the steps of: introducing a positiveelectrolyte from said redox flow battery into a negative electrolytechamber having a negative electrode, introducing a formic acid solutioninto a positive electrolyte chamber having a positive electrode andbeing connected to said negative electrolyte chamber through a membrane,applying a voltage between said positive electrode and said negativeelectrode for emitting electrons from said formic acid solution forchanging a portion of metal ions having a high valence state containedin said positive electrolyte, to metal ions having a low valence state,and returning, to said redox flow battery, said positive electrolytecontaining metal ions of said low valence state.
 7. The method forregenerating electrolyte according to claim 6, further comprising thesteps of: returning said formic acid solution from which said electronsare emitted, from said positive electrolyte chamber, to a formic acidsolution tank, and separating carbon dioxide gas from said returnedformic acid solution.
 8. The method for regenerating electrolyteaccording to claim 6, further comprising the steps of: circulating saidformic acid solution between said positive electrolyte chamber and saidformic acid solution tank for storing said formic acid solution, andnewly supplying formic acid solution of a predetermined concentration tocompensate for an amount of said formic acid solution consumed in saidformic acid solution tank.
 9. The method for regenerating electrolyteaccording to claim 6, wherein said formic acid solution is a watersolution containing formic acid of 40 wt.% or more.
 10. The method forregenerating electrolyte according to claim 9, wherein said formic acidsolution is a water solution containing formic acid of 80 wt.% or more.11. The method for regenerating electrolyte according to claim 6,wherein said metal ions are iron ions.
 12. The method for regeneratingelectrolyte according to claim 6, wherein said negative electrolyte ofsaid redox flow battery includes Cr³⁺ ions and Cr²⁺ ions.
 13. The methodfor regenerating electrolyte according to claim 6, wherein said stepsare executed when an operation of said redox flow battery is stopped.14. The method for regenerating electrolyte according to claim 6,wherein said steps are executed while said redox flow battery isoperated.